Monostable multivibrator with bias applied to tap of variable rc network for linearization of chargetime



June 8, 1965 GABOR ETAL 3,188,498

MONOSTABLE MULTIVIBRATOR WITH BIAS APPLIED To TAP OF VARIABLE RC NETWORKFOR LINEARIZATION OF CHARGE-TIME Filed Aug. 8, 1965 United States PatentOfiice 3,183,493 Patented June 8, 1965 3,188,498 MONOSTABLE MULTWHBRATGRWITH BIAS APPLlED T TAP 0F VARIABLE RC NET- ;XORK FOR LINEARIZATION OFCHARGE- ME Andrew Gabor and Janos T. Barany, Port Washington, N.Y.,assignors to Potter Instrument Company, Inc, Plainview, N .Y., acorporation of New York Filed Aug. 8, 1963, Ser. No. 300,842 7 Claims.(Cl. 307-4585) This invention relates to a monostable multivibrator and,more particularly, relates to such a multivibrator in which improvedcircuitry is provided to change the time constant of the state changeand thus the duration of the single shot output pulses.

A monostablemultivibrator is, of course, known to the art. Insuch adevice, a single input trigger will generate an 'output pulse, theduration of which is determined by the time constant of an RC circuitcoupling the sections of the multivibrator.

In many applications, it is desirable to provide means for changing thetiming of the multivibrator and in such applications, the art hasprovided a variable resistor or potentiometer in the RC timing circuit.By change of the resistance in the timing circuit, the time constantand, thus,the output pulse width of the multivibrator can be changed.

However, in those applications in which a wide variation in timing isdesired, certain difficulties are encountered with this type ofarrangement. First, when large changes in resistance is necessary, thepotentiometer inherently makes it difficult to adjust the timing withthe desired resoliition over the operating range since the rate ofchange of resistance per degree of movement of the tap electrode 'isnota constant in a linear potentiometer (the only practical type forlarge resistance changes). Secondly, since the RC circuit includes thedropping resistor between the source and the base of one section of themultivibrators, wide variations in resistance will often causeover-driving of the base.

It is, therefore, the primary object of this invention to provide animproved monostable multivibrator in which the eifective time constantof the circuit can be changed over a wide range and with high resolutionover the entirerange.

Other objects and advantages of the invention will be pointed out in thefollowing detailed description which may best be understood by referenceto the accompanying figure which is a schematic diagram of amultivibrator constructed in accordance with the present invention.

In the figure, there is shown a multivibrator consisting of transistorswand 11, each of which is provided with a base, emitter and collectorelectrode 14, 16, 18 and 15, 17, 19, respectively. The multivibrator isconnected in the usual manner by grounding the emitter electrodes 16 and17. The collector electrode 19 is coupled to source 20 through resistor22 and is tied back to the base electrode 14- through resistor 24 whichmay be bypassed by capacitor 26. The collector electrode 18 is coupledthrough circuit 28 and the timing capacitor 30' to the base electrode 15of transistor 11. The base electrode 15 is coupled through resistor 32to terminal 34. A potentiometer 36 is provided, the end terminals beingcoupled between terminals 34 and 38 and the tap electrode 49 thereofbeing coupled to source 20. Resistor 42 is provided between junction 38and ground. Terminal 38 is also coupled through circuit 44 and thecollector resistor 46 to the collector 18 of the transistor 10. Apositive supply voltage source 21 is connected through resistor 48. Thecircuit 28 is merely an emitter follower comprising a transistor 50coupled to an emitter follower circuit to make the junction 52 a lowimpedance point. Similarly, circuit 44 is an emitterfollower to makejunction 38 a low impedance point. Aside from the problems of impedancematching, these circuits should be omitted. For example, circuit 28 isconventionally included in this point to make point 52 a low impedancepoint thereby toimprove the recovery characteristics and recovery speedof the circuit without, however, influencing the circuit operation inother aspects.

In operation, the transistor 10 is cut off and transistor 11 issaturated. An input pulse as, for example, applied over lead 54 wouldbe'a negative-going pulse to bring the base 14 of transistor 10negatively starting conduction of transistor 10. As a result, junctionSZwill go, positively swinging base 16 positively to decrease theconduction in transistor 11, in turn swinging collector 1? negatively tofurther increase the current of transistor 10. As is well known, thisswing is very fast and the circuit reaches a stable point withtransistor '10 conducting. Conduction is continued during the dischargeof the timing capacitor 30 through resistor 32 and the portion ofpotentiometer 36 between the tap 40 and terminal 34. Thus, the tapposition varies the R of the timing circuit.

As the timing capacitor 30 discharges through the resistors 32 and theportion 36 and the low saturation resistance of the conductingtransistor 10, the base 15 of transistor 11 becomes less positive.When'the base potential becomes slightly negative, transistor 11 againconducts. The collector potential 19increases positively and since thispotential is coupled to the base 14 of transistor 10, drives transistor10 to cut off. Thus, transistor 10 will again be cut off and transistor1-1 will conduct in saturation with its collector voltage substantiallyat zero. This stable condition is maintained until another pulsetriggers the circuit.

The adjustment of the timing by movement of the tap 40 on. potentiometer36 is the result of two efiects. The first effect of tap movement isvariation in the resistance serially connected to the timing capacitor.This variation of tap position changes the RC time constant of the timing circuit with concomitantchanges in the duration of the output pulse.The second effect of movement of tap position is variation of thevoltage applied across the collectorresistor 46, thus, varying the swingof potential across the resistor 12 by the drop through thepotentiometer resistance between the tap position 40 and terminal 38thereof.

For example, if the tap ttl is moved to terminal 34 at the extreme endof the potential, the time constant of the timing circiut will be thelowest since the resistance is the lowest. Simultaneously, the voltageacross resistor 46 is the lowest and the width of the pulse output asindicated by wave form 56 derived from the collector of transistor 11 bylead 58 will be the shortest.

If, on the other hand, the tap is moved to the extreme end ofpotentiometer 36 by movement to terminal 38, the maximum resistance willbe introduced into the timing circuit and at the same time, the highestvoltage will be applied across collector resistor 46 to give the highestvoltage swing thereacross and the lowest time of the pulse output.

The combination of the two effects linearizes the potentiometer so thatthe rate of change of time bears 'a substantially linear relationship tothe rate of change of movement of the tape position.

By way of complete disclosure, but not by way of limitation, thecomponent values listed in Table I are typical of an operating circiutconstructed in accordance with the present invention.

3 Table I Component Number Component Identification o p13. 1 mierofarad.

This invention may be variously modified and embodied within the scopeof the subjoined claims.

What is claimed is:

1. A monostable multivibrator having,

a first and second transistor,

each of said transistors having at least a base, collector and emitterelectrodes,

a timing capacitor coupling the collector electrode of said firsttransistor to the base electrode of said second transistor,

a biasing source,

a potentiometer having a first and second terminal and a tap electrodeadjustably positioned between said first and second terminal,

means coupling said biasing source to said tap electrode,

a first resistor coupling said base electrode of said second transistorto said first terminal of said potentiometer,

a second resistor coupling said collector electrode of said firsttransistor to said second terminal of said potentiometer,

a third resistor coupling said second terminal of said potentiometer toground,

whereby movement of said tap electrode simultaneously changes theeffective resistance in the timing circuit including said timingcapacitor and changes the potential applied to the collector electrodeof said first transistor.

2. A monostable multivibrator in accordance with claim 1 which includesan emitter follower coupled between the collector electrode of firsttransistor and said timing capacitor to improve the recovery speed ofthe multivibrator.

3. A monostable multivibrator in accordance with claim 1 which includesan emitter follower coupled between said second resistor and said secondterminal of said potentiometer to make said second terminal a lowimpedance point in said circuit.

4. A monostable multivibrator in accordance with claim 1 in which saidtransistors are PNP type transistors and in which said source is asoruce of negative voltage.

5. A monostable multivibrator having an adjustable timing of the outputpulse therefrom, which comprises,

a first and second transistor, 7

each of said transistors having a base, collector and emitterelectrodes,

rneans coupling said base electrodes of said first and secondtransistors to ground,

a first resistor coupling said base electrode of said first transistorto said collector electrode of said second transistor,

a capacitor coupled across said first resistor,

a timing capacitor coupling said collector electrode of of said firsttransistor to the base electrode of said second transistor,

a bias source,

a potentiometer having first and second terminals and a tap electrodeadjustably positioned between said first and second terminals,

means coupling said bias source to said tap electrode,

a first collector resistor-coupling the collector of said firsttransistor to said second terminal of said potentiometer,

a second collector resistor coupling the collector electrode of saidsecond transistor to said bias source,

a timing resistor coupling said base electrode of said second transistorto said first terminal of said potentiometer,

resistor means coupling said second terminal of said potentiometer toground,

a second bias source of opposite polarity and resistor means couplingsaid second source to the base electrode of said first transistor,

whereby movement of said tap electrode will vary the time of the outputof said multivibrator linearly with movement of said tap electrode.

6. A monostable multivibrator in accordance with claim 5 which includes,

a third transistor having a base, emitter and collector electrodes,

a third collector resistor coupling the collector electrode thereof tosaid bias source,

an emitter resistor coupling said emitter electrode of said thirdtransistor to ground,

means coupling said first collector resistor tothe emitter electrode ofsaid third transistor, and

means coupling said base electrode of said third transistor to saidsecond terminal of said potentiometer.

'7. A monostable multivibrator in accordance with claim 5 whichincludes,

a fourth transistor having a base, collector and emitter electrodes,

a fourth collector resistor coupling said collector electrodes to saidbias source,

a second emitter resistor coupling said emitter electrode of said fourthtransistor to said second bias source,

means coupling said emitter electrode to said timing capacitor,

means coupling said base electrode to the collector electrode of saidfirst transistor, and I a diode, said diode being coupled between saidbase and emitter electrodes of said fourth transistor.

References Cited by the Examiner UNITED STATES PATENTS 2,479,954 8/49Moore 328-207 X 2,976,427 3/61 Armanini 307-885 3,067,342 12/62 Waller30788.5

ARTHUR GAUSS, Primary Examiner,

1. A MONOSTABLE MULTIVIBRATOR HAVING, A FIRST AND SECOND TRANSISTOR,EACH OF SAID TRANSISTORS HAVING AT LEAST A BASE, COLLECTOR AND EMITTERELECTRODES, A TIMING CAPACITOR COUPLING THE COLLECTOR ELECTRODE OF SAIDFIRST TRANSISTOR TO THE BASE ELECTRODE OF SAID SECOND TRANSISTOR, ABIASING SOURCE, A POTENTIOMETER HAVING A FIRST AND SECOND TERMINAL AND ATAP ELECTRODE ADJUSTABLY POSITIONED BETWEEN SAID FIRST AND SECONDTERMINAL, MEANS COUPLING SAID BIASING SOURCE TO SAID TAP ELECTRODE, AFIRST RESISTOR COUPLING SAID BASE ELECTRODE OF SAID SECOND TRANSISTOR TOSAID FIRST TERMINAL OF SAID POTENTIOMETER, A SECOND RESISTOR COUPLINGSAID COLLECTOR ELECTRODE OF SAID FIRST TRANSISTOR TO SAID SECONDTERMINAL OF SAID POTENTIOMETER, A THIRD RESISTOR COUPLING SAID SECONDTERMINAL OF SAID POTENTIOMETER TO GROUND, WHEREBY MOVEMENT OF SAID TAPELECTRODE SIMULTANEOUSLY CHANGES THE EFFECTIVE RESISTANCE IN THE TIMINGCIRCUIT INCLUDING SAID TIMING CAPACITOR AND CHANGES THE POTENTIALAPPLIED TO THE COLLECTOR ELECTRODE OF SAID FIRST TRANSISTOR.